Nonwoven fabrics may be produced by hydroentangling webs of fibers with high energy water jets as described in U.S. Pat. No. 3,485,706 (Evans et al). Hydroentangled nonwovens have been used for disposable rags, outer cover and liner materials for absorbent products, as substrates for wet wipes, and for various other single-use disposable, and multiple-use applications.
Various fiber types have been successfully hydroentangled. Short fibers, such as wood pulp, recycled fibers, and cotton linters have been hydroentangled, sometimes with the aide of a scrim or long fiber matrix. Longer, staple length fibers are also known to be amenable to the hydroentangling process, including polyesters, cotton staple, polyamides, polyacrylates, and polyolefins. Among the polyesters, polyethylene terephthalate, aliphatic-aromatic co-polyesters, polyhydroxyalkanoates (PHA), and polylactide (PLA or polylactic acid) have been hydroentangled. Fabrics comprising continuous filaments, such as spunbond nonwoven fabrics, are also known to be suitable for hydroentangling.
EP 1 226 296 B1 (Fingal et al) discusses heating polymer fibers at the moment of hydroentangling to reduce the flexural rigidity of the fibers and achieve a higher degree of entanglement in the finished fabric. Fingal et al; reported that the increased entanglement was reflected in greater tensile strength when the fabric was tested in surfactant solution.
Hydroentangled nonwoven fabrics are often chosen because of their lower cost, relative to knitted or woven fabrics. To reduce the cost of manufacturing hydroentangled nonwoven fabrics it is desirable to operate the production line at high speed.
One difficulty in hydroentangling certain synthetic fibers is their high wet stiffness, i.e. modulus, compared to wet cellulosic fibers. The stiffness of some synthetics may result in inefficient fiber entanglement, resulting in poor tensile properties of the finished fabrics.
While operating a nonwoven fabric production line at high speed, one aspect is that the fabric is likely to be subjected to high tension as it is transported along the production line. There is a tendency for nonwoven fabrics to “neck” when pulled. This problem is especially severe for soft polymers that are subject to distortion under tension. Necking is the tendency for the fabric to stretch in the direction of tension (usually the machine direction or MD), while contracting in the perpendicular direction (cross machine direction or CD). Furthermore, the fabrics tend to distort non-uniformly, becoming more stretched along the median than along either edge. Such a distorted sheet of fabric is difficult handle, form into neat rolls and subsequently convert into finished products.
Various solutions to the problem of necking fabrics problem have been attempted. One solution is to use tenter frames, as discussed in U.S. Pat. No. 4,788,756 (Leitner). A tenter frame applies tension to the fabric in the CD, thus limiting necking. Tenter frames have limited utility in high speed operations and tend to be mechanically complex, subject to break down, and cause damage to the selvage.
A second approach to limit necking is to transport fabrics under a minimum of tension. To minimize tension on the fabric, it is transported on screens, drums, or belts and the equipment is gradually and evenly accelerated each time the production line starts up. This approach is widely used in manufacturing, but there inevitably are sections in the production line where the fabric is unsupported; and even with sensors and computer controls, a gradual, even acceleration is difficult to accomplish.
In view of the above, a need currently exists for a high speed, inexpensive, reliable method of processing stiff fibers into hydroentangled nonwoven fabrics and minimizing necking. The fabrics made by this process may be used for components of absorbent disposable products, wipers, and other applications.